1. Field
The present disclosure relates generally to inspecting components and in particular to a method and apparatus for identifying variations in a component. Still more particularly, the present disclosure relates to a method and apparatus for nondestructive inspection of interwoven wire fabrics.
2. Background
Today's aircraft are being designed and built with greater percentages of composite materials. In some aircraft, up to fifty percent of the structural components are being manufactured with composite materials. Composite materials are tough, light-weight materials. These types of materials may be made by combining two or more dissimilar products, such as fibers and resins, to create a product with improved or exceptional structural properties not present in the original materials.
Composite materials are used in aircraft to meet goals, such as reducing weight and increasing payloads in the aircraft. Further, composite materials also are used because these types of materials have improved fatigue, life, and increased corrosion resistance as compared to other currently used materials.
Although composite materials are lighter and have better mechanical and fatigue properties as compared to aluminum, these types of materials are less electrically conductive and have poor electromagnetic shielding. These features cause poor current dissipation when an electromagnetic effect, such as a lightning strike occurs. Further, composite materials are subject to greater damage due to lightning strikes than traditional aluminum materials used in aircraft.
Specifically, when lightning hits an aircraft, a conductive path on the skin of the aircraft allows the electricity to travel along the skin and exit at some other location on the aircraft. Without an adequate conductive path, arcing and hot spots can occur. These types of effects may char, delaminate, and/or penetrate the skin of the aircraft. As a result, the load bearing characteristics of the structure of the aircraft may be returned. Thus, the lower electrical shielding capability of composite materials increases the likelihood that circuits within the aircraft may be affected by the lightning strike.
One current mechanism used to protect composite skins on aircraft against lightning strike damage, is to include conductive lightning skin protection systems. These types of systems may be present either in or on the composite skins of an aircraft. One type of system used to provide a conductive path on the aircraft is an interwoven wire fabric. With this type of system, wires, such as phosphor-bronze wires are embedded in the top layer of the composite material nearest the wind swept surface. This type of material is commonly used in the fuselage of an aircraft. Other types of systems may include the use of a thin copper foil, such as on the wings of an aircraft using composite materials.
With an interwoven wire fabric system in the fuselage, the wires typically have a thickness range of about 0.003 to about 0.004 inches. Further, these types of wires are spaced apart from each other. The spacing is around one tenth of an inch in a ninety degree mesh pattern.
In inspecting installations of composite skins containing interwoven wire fabric, it is desirable to be able to have these wires not more than a selected distance below the surface of the skin. This distance between the surface and the skin is for economic reasons, and not safety reasons. With respect to economic issues, it is more expensive if the distance to the surface of the skin from the wires is too great. The economic costs are larger with the greater distance because increased damage to layers above the interwoven wire fabric may occur. For example, damage to this layer and layers such as paint may increase with lightning strikes as compared to a smaller distance from the surface of the skin to the wires.
Additionally, it is desirable to determine if bonded repairs on composite panels containing interwoven wire fabrics have a sufficient overlap between the patch material and the current material. The overlap width should be sufficient to allow the transfer of energy from a lightning strike on a bonded repair section into the parent material of the fuselage. The overlap width is the width of the edge of the patch material extends over the parent material. The width is typically about an inch. At the overlap, the repair fabric is on top of the parent material, so that the fabric is essentially doubled up in this region. This width is constant all around the patch area. In inspecting new installations and repairs, it is desirable to be able to make these types of inspections in a nondestructive manner.